A low-noise amplifier (LNA) is an electronic amplifier used to amplify very weak signals, for example radio frequency (RF) signals captured by an antenna. In an exemplary application, an LNA is included in a front-end of an AM radio receiver. Using an LNA, the effect of noise from subsequent stages of the AM radio receiver is reduced by the gain of the LNA, while the noise of the LNA itself is injected directly into the received signal. Thus, it is necessary for an LNA to boost the desired signal power while adding as little noise and distortion as possible. This is accomplished by operating the LNA in its linear region.
Automatic gain control (AGC) is an adaptive means for adjusting the gain to an appropriate level for a range of input signal strength levels. In an AM radio receiver, the received signal strength varies from a weak to a strong signal. Without AGC, the sound emitted from the AM radio receiver varies according to the received signal strength. The AGC circuit detects the overall strength of the signal and automatically adjusts the gain to maintain an approximately constant average output level. For a very weak signal the AGC has no effect; as the signal increases, the AGC reduces the gain.
In order for the LNA to maintain operation in the linear region, input signals having too high a signal strength must be attenuated. Without such attenuation, input signals of sufficient signal strength overload the LNA, driving the LNA into a non-linear region and distorting the resulting amplified signal. In general, signal attenuation should reduce the amplitude or power of a signal without appreciably distorting the signal waveform.
As applied to an RF signal input to an LNA in an AM radio receiver, an objective is to vary the gain of the LNA by means of a variable attenuator in such a way that the RF signal input to the LNA is not distorted. Only the RF signal amplitude should be scaled, without creating any distortion product.
The variable attenuation function found in conventional AM radio receiver front-ends is accomplished using a PIN diode as a variable resistor coupled to the input of the LNA. FIG. 1 illustrates an exemplary schematic diagram of a conventional AM radio receiver front-end. The PIN diode functions as a current controlled variable resistor. As the current is increased through the current regulator, the resistance of the PIN diode varies from several megohms at zero current to a relatively low value, such as a few tens of ohms, at 10 mA. As the resistance of the PIN diode decreases, more of the RF signal received by the antenna is shunted from the input of the LNA. In this sense, the variable resistance PIN diode functions as a signal attenuator. The variable resistance PIN diode reduces the input RF signal to prevent overloading of the LNA. Without the variable resistance PIN diode, an RF signal of sufficient signal strength may overload the LNA, driving the LNA into a non-linear region that results in distortion of the amplified signal.
Although effective in providing signal attenuation, the variable resistance PIN diode is an external component that is not part of the radio receiver integrated circuitry resulting in increased cost. Also, operation of the variable resistor PIN diode requires a significant amount of DC current. As such, the variable resistance PIN diode suffers from cost and power consumption issues. An alternative approach to the variable resistance PIN diode is to vary the gain of the LNA directly by changing the bias current in the LNA. However, this approach introduces distortion within the LNA.